Bring a large pot of water to a boil; add the spaghetti and cook according to
package directions.

Spray a 2-quart casserole with nonstick cooking spray. Place the frozen spinach
in the casserole; microwave, covered, on high for 5 minutes. Stir and add the
garlic; microwave, covered, on high for 2 minutes.

Drain the spaghetti and immediately add it to the casserole along with the
mozzarella and Parmesan cheeses. Stir to combine and microwave, covered, on high
for 2 minutes, or until heated through. Serve immediately.